A) FIELD OF THE INVENTION
The present invention relates to an optical system including a liquid crystal lens and/or a liquid crystal stop device which is suitable for endoscopes.
B) DESCRIPTION OF THE PRIOR ART
An optical system for endoscopes is constructed so that light is irradiated toward an object and an image of the object is formed by the light reflected from the object, and it has recently been proposed that liquid crystal elements such as liquid crystal lenses and liquid crystal stops should be used in the optical system to eliminate mechanical driving sections for focusing and exposure control.
By way of example, a liquid crystal lens, as shown in FIG. 1, comprises a liquid crystal cell 1 and a polarizing plate 2 arranged in front thereof (i.e., on an object side), in which the liquid crystal cell 1 is constructed in such a way that a transparent electrode 4 and an orientation film 5 are provided on each of surfaces, directed to each other, of transparent plates 3, made from materials such as glass or acrylic resin, at least one of which is curved, and a nematic liquid crystal 6 is enclosed in a positive lens-shaped cell configured by the surfaces directed to each other. In the case where a switch SW is turned off and a voltage is not applied across the transparent electrodes 4, molecules of the liquid crystal 6 assume the homogeneous alignment that a direction of a longitudinal axis of each molecule arrayed by the orientation film coincides with a vibrating direction of the polarizing plate 2. Accordingly, the liquid crystal 6 will reach a high state of its refractive index with respect to incident light transmitted by the polarizing plate 2, thus reducing the focal length of the liquid crystal lens. Further, where the switch SW is set to ON and the voltage more than a constant value is applied between the transparent electrodes 4, the molecules of the liquid crystal 6 assume the homeotropic alignment, namely, the alignment that the direction of the longitudinal axis of each molecule is normal to the vibrating direction in a plane of vibration of the polarizing plate 2, with the result that the refractive index of the liquid crystal 6 relating to the incident light diminishes and the focal length of the liquid crystal lens increases.
Also, as an example of the liquid crystal stops, the structure of the liquid crystal lens making use of a twisted nematic (TN) liquid crystal cell is shown in FIG. 2. A TN liquid crystal cell 7 comprises a nematic liquid crystal 11 enclosed in a cell configured so that two faces, on which transparent electrodes 9, and orientation films 10, are laminated to transparent substrates 8, are directed to each other in such a manner that an orientation direction is twisted at an angle of 90.degree., in which one of the transparent electrodes 9, as shown, is configured into an annular shape devoid of its middle portion to assume a variable stop. The liquid crystal cell 7 is then sandwiched between two polarizing plates 12, 13 whose polarizing directions are normal to each other so that the liquid crystal stop is constructed. Since the molecules of the liquid crystal exhibit the twist alignment in an off condition of the switch SW, linearly polarized light incident on the TN liquid crystal cell 7 through the polarizing plate 12 traverses and emanates from the polarizing plate 13 after its plane of polarization is rotated at an angle of 90.degree. by the liquid crystal cell 7. However, when the switch is set to ON and a voltage more than a constant value is applied between the transparent electrodes 9, the liquid crystal molecules are arrayed along an electric field to turn to the homeotropic alignment and lose the function of rotating the plane of polarization, with the result that the light traversing the TN liquid crystal cell 7 cannot pass through the polarizing plate 13. Since such an effect is not brought about in a place devoid of the transparent electrode 9, it follows that, in the middle portion of the liquid crystal stop, the light always passes irrespective of the on-off operation of the switch SW, while in the annular portion of its outside, the changeover of block-transmission of light is performed in response to the on-off operation of the switch SW.
In FIG. 3, the structure is depicted in which the liquid crystal stop is provided in such a manner that the center of a middle circle 9a of the transparent electrode 9 coincides with an optical axis of the optical system for endoscopes. This drawing depicts the optical system of a tip portion of a fiber scope, in which an arrangement is such that an objective lens unit 14 comprises a retrofocus-type lens system provided with a negative lens 15 on an object side and a positive lens group including positive lenses 16, 17, 18 provided on its exit side and an entrance end face of an image guide fiber bundle 19 is disposed in close vicinity to an exit surface of the positive lens 18. The liquid crystal stop is provided in such a position that imaging beams of light are nearly parallel to each other between the positive lenses 16 and 17. A stop 20 placed directly behind the liquid crystal stop consists of a light blocking plate provided with a circular aperture by which the maximum aperture of the objective lens unit will be defined. The positive lens 18 acts as a field lens for making a principal ray incident almost vertically on the entrance end face of the image guide 19. Thus, when the switch SW is set to OFF, the F-number of the objective lens unit 14 will be determined by the aperture stop 20, while on the other hand, when the switch SW is ON, it will be determined by the middle circle 9a of the transparent electrode 9 of the liquid crystal cell 7, and as such the optical system for endoscopes variable in F number can be obtained.
Also, since the liquid crystal stop is usually arranged perpendicular to the optical axis, as shown in FIG. 3, the problem arises that the contrast between transmission and blockage of light changes in accordance with an angle of incidence of light, due to angle dependency of an electrooptic effect of the liquid crystal. FIG. 4 shows that when the thickness of the liquid crystal cell 7 of the liquid crystal stop is 10 .mu.m, the frequency of a power source P is 100 H.sub.z, and the temperature of the liquid crystal cell 7 is 30.degree. C., the contrast between transmission and block of light of the liquid crystal stop, viewed from a direction making an angle of 30.degree. with the optical axis as shown in FIG. 5, changes over the entire circumference. Also, in the liquid crystal cell of FIG. 5, it is assumed that the aperture 9a of the transparent electrode such as is shown in FIG. 3 is not provided. In FIG. 4, the circumference represents angles from a reference position, the radii indicate contrast, and the contrast becomes progressively high in separating from the center of the circle. As will be apparent from this diagram, the contrast is not symmetrical with respect& to the optical axis, lowest when viewed from the upper side at an angle (90.degree.), highest when viewed from the lower side at an angle (279.degree.), and moderate at other angles. Accordingly, in the example of FIG. 3, the problem is encountered that the effect of the stop varies in response to the position of a visual field in such a manner that since a beam of light A exhibits low contrast as compared with a light beam being oblique at the same angle on the opposite side thereof, even when the switch SW is set to ON, part of the light beam A will pass through the annular portion, while the light beam inclined on the opposite side is completely blocked.
In general, a polarized light component (P component) vibrating in a plane parallel with a reflecting surface and a polarized light component (S component) having a vibrating direction normal to the P component are different in reflectance from each other. As a result, in the case where the liquid crystal device such as is stated above is used in the optical system for endoscopes, an improper selection of the polarizing direction of the polarizing plate has caused the problems that specular reflection light prevents observation as a bright spot and an observation image with average intensity of the P component and the S component is not attained.
Moreover, the liquid crystal device such as stated above has the properties that the speed of change of the state by the on-off operation of the switch SW varies in response to the temperature condition of the liquid crystal per se. Specifically, the speed of change decreases at low temperatures, while it increases at high temperatures. Where the liquid crystal device of this type is utilized in the optical system for endoscopes, the temperature of the liquid crystal is principally affected by an ambient temperature in the distal end portion of the endoscope and the heat generated by the illuminating light.